1. Field of the Invention
The invention relates to an absorption spectrometer for measuring the concentration of a gaseous component of interest in a measurement gas.
2. Description of the Related Art
WO 2012/109030 A1 discloses an absorption spectrometer and method. Laser spectrometers are employed for optical gas analysis, e.g., in process measurement technology. A laser, as a rule a laser diode, generates light in the infrared region in this respect, which is routed through a gas to be measured (process gas) and then detected. The wavelength of the light is tuned to a specific absorption line of the respective gaseous component to be measured, to which end the laser diode is usually activated periodically with a sawtooth or triangular current signal to periodically scan the absorption line with the generated light in a wavelength-dependent manner.
In the case of direct absorption spectroscopy, the concentration of the gaseous component of interest is determined immediately from the reduction in light intensity (absorption) detected at the location of the absorption line. In the case of wavelength-modulation spectroscopy, the wavelength of the generated light is additionally modulated sinusoidally with a comparatively high frequency in the kHz range and with small amplitude, during the comparatively slow wavelength-dependent scanning of the absorption line. The profile of the absorption line is not linear. As a result, higher-order harmonics are generated in the detector signal or measurement signal. The measurement signal is demodulated in the case of a higher-order harmonic of this type, preferably the second harmonic, by a phase-sensitive lock-in technique, and analyzed to produce a measured result for each scanning interval. The analysis is effected, for example, by curve-fitting the demodulated measurement signal profile to be expected in the ideal case and as described analytically by using an approximation model (target curve) to its actual profile (actual curve). Since one of the parameters of the approximation model is proportional to the concentration of the gaseous component, what is obtained from the analysis and therefore the measured result is the concentration of the gaseous component to be measured.
Wavelength-modulation spectroscopy is particularly advantageous for measuring low concentrations because it is better able to filter out noise from the measurement signal. At higher concentrations, however, the approximations necessary for the analysis of the measurement signal become increasingly inaccurate, with the result that measurement error rises. In the case of absorption spectroscopy, on the other hand, measurement error is higher due to the higher noise sensitivity in the case of small concentrations. But since an approximation description of the absorption line is not necessary, measurement accuracy becomes better with increasing concentration as the useful signal then also becomes stronger.
The size of the measurement signal is inversely proportional to the absorption of the light on the path from the laser to the detector. For its part, the absorption at the location of the specific absorption line shows a monotonic dependency in accordance with the Lambert-Beer law, which is approximately proportional during most measurement tasks, to the product of the concentration of the gaseous component of interest and the length of the light path between the laser and the detector. The lower the concentrations to be measured, the longer the absorption distance has to be in order to obtain an adequately large measurement signal. Whereas in the case of in-situ measurements, long absorption distances are present as a rule because of the constructional nature of the process plant (e.g. smokestack in an incineration plant), the challenge for extractive measurements, where the measurement gas is guided through a gas cell situated between the laser and the detector, is to create a long absorption distance over a small space. This is usually done by using a multi-reflection gas cell, such as a Herriott or White cell, in which the optical path length and therefore the absorption distance is enlarged by means of multiple reflection of the light between mirrors. A weakness of gas cells of this type, however, is their sensitivity with reference to environmental influences such as temperature fluctuations or vibrations. Thus, relatively small changes in geometric parameters, such as the laser's beaming angle or the angles and spacings of the mirrors, can result in major changes in the optical path length and therefore the absorption distance, particularly if the quantity of reflections varies as a result.